In general computed tomography, tomographic images of an examination object, in particular of a patient, are taken with the aid of absorption measurements of X-rays that penetrate the examination object, a radiation source generally being moved in the shape of a circle or spiral about the examination object, and for the most part, a multirow detector with a multiplicity of detector elements, measuring the absorption of the radiation upon passage through the examination object on the side of a detector opposite the radiation source. For the purpose of tomographic imaging, tomographic slice images or volume data are reconstructed from the measured absorption data of all the measured spatial rays. Very fine absorption differences in objects can be displayed with the aid of these computed tomography images, but zones of similar chemical composition that naturally also have a similar absorption behavior are displayed only with unsatisfactory detail.
It is known, furthermore, that the effect of the phase shift upon passage of a beam through an examination object is substantially stronger than the absorption effect of the material penetrated by the radiation. Such phase shifts are known to be measured by the use of two interferometric gratings. These interferometric measuring methods are referred to, for example, in “X-ray phase imaging with a grating interferometer, T. Weitkamp et al., Aug. 8, 2005/Vol. 12, No. 16/OPTICS EXPRESS”.
In the case of this method, an examination object is irradiated by a coherent X-radiation and subsequently guided through a pair of gratings, and the radiant intensity is measured directly after the second grating. The first grating produces an interference pattern that images a moiré pattern on to the detector lying therebehind with the aid of the second grating. If the second grating is slightly displaced, this likewise results in a displacement of the moiré pattern, that is to say a change in the spatial intensity in the detector lying therebehind, which can be determined relative to the displacement of the second grating. If the change in intensity is plotted for each detector element of this grating that is to say for each beam, as a function of the displacement path of the second grating, the phase shift of the respective beam can be determined. The fact that this method requires a very small radiation source is a problem, and therefore cannot be applied in practicing computed tomography of relatively large objects, since formation of the interference pattern requires a coherent radiation.
In one possibility, the method shown in the abovenamed document requires a radiation source with an extremely small focus such that a sufficient degree of spatial coherence is present in the radiation used. However, when such a small focus is used there is then, in turn, an insufficient dose rate for examining a relatively large object. However, there is also the possibility of using a monochromatically coherent radiation, for example, a synchrotron radiation, as radiation source, but the construction of the CT system is thereby rendered very expensive and so a widespread application is impossible.
This problem can be circumvented by arranging a first absorption grating inside the focus/detector combination in the beam path, directly following the focus. The alignment of the grating lines is in this case parallel to the grating lines of the interference grating following the examination object.
The slits of the first grating produce a field of individually coherent beams that suffices for producing the interference pattern known per se with the aid of the phase grating arranged downstream of the object in the beam direction. It is possible in this way to use radiation sources that have dimensions corresponding to the normal X-ray tubes in CT systems or transmitted light X-ray systems such that, for example, it is now also possible to make well differentiated soft part images in the field of general medical diagnostics with the aid of X-ray machines. Reference is made in this regard to the German patent applications, which are not prior publications, having the file numbers 10 2006 017 290.6, 10 2006 015 358.8, 10 2006 017 291.4, 10 2006 015 356.1 and 10 2006 015 355.3, the entire disclosure content of each of which is hereby incorporated herein by reference.
The use of such X-ray optical gratings in conjunction with X-ray CT systems is, however, very demanding technically since these X-ray optical gratings require structures with a very high contrast ratio, for example 100 μm, and at the same time require a very short period of the order of magnitude of 2 μm, corresponding to web widths of approximately 1 μm. Moreover, the aim is to use strongly absorbing material for these gratings, ideally to use gold. At the same time, it is described in the abovenamed documents that the data obtained from the phase contrast measurement are also available for the absorption tomographic imaging. However, the problem thereby arises that this leads to a strong radiation burden on the patient because of the required strong absorption of the X-ray gratings. Consequently, such CT systems with permanently installed X-ray optical gratings in the beam path between the patient and the detector system cannot also be used regularly for absorption CT.